Chemical-mechanical planarization ("CMP") processes remove material from the surface of semiconductor wafers or other substrates in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 30, and a polishing pad 40. The polishing pad 40 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be an abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 44 may be a conventional CMP slurry with abrasive particles and chemicals that remove material from the wafer, or the planarizing liquid 44 may be a planarizing solution without abrasive particles. In most CMP applications, conventional CMP slurries are used on conventional polishing pads, but planarizing solutions without abrasive particles are used on abrasive polishing pads.
The CMP machine 10 also has an under pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 40. A drive assembly 26 rotates the platen 20 (as indicated by arrow A), or it reciprocates the platen back and forth (as indicated by arrow B). Since the polishing pad 40 is attached to the under pad 25, the polishing pad 40 moves with the platen 20.
The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier; or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and/or rotational motion (indicated by arrow C and arrow D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 30 presses the wafer 12 face-downward against the polishing pad 40, and at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the wafer 12 across the planarizing surface 42. As the face of the wafer 12 moves across the planarizing surface 42, the polishing pad 40 and/or planarizing solution 44 continually remove material from the face of the wafer 12.
CMP processes must consistently and accurately produce a uniform planar surface on the wafer to enable precise circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 .mu.m. Focusing photo patterns to such small tolerances, however, is difficult when the planarized surface of the wafer is not uniformly planar. Thus, CMP processes must create a highly uniform planar surface.
One problem with the CMP processes is that the surface of the wafer may not be uniformly planar because the rate at which the thickness of the wafer decreases (the "polishing rate") may vary from one area of the wafer to another. The polishing rate depends, in part, on the relative linear velocity between the surface of the wafer and the portion of the planarizing surface contacting the wafer. The linear velocity of the planarizing surface of a circular, rotating polishing pad varies across the planarizing surface of the pad in proportion to the radial distance from the center of the pad. Similarly, the linear velocity also varies across the front face of the wafer in proportion to the radial distance from the center of the wafer. The variation of linear velocities across the face of the wafer and planarizing surface of the polishing pad creates a relative velocity gradient between the wafer and the polishing pad. In general, the relative velocity gradient between the wafer and the pad causes the polishing rate to vary across the face of the wafer in a center-to-edge profile where the perimeter of the wafer polishes faster than the center of the wafer.
Several devices and concepts have been developed to reduce the center-to-edge planarizing profile across wafers. For example, U.S. Pat. No. 5,020,283 to Tuttle discloses a non-abrasive polishing pad with voids in the surface of the pad. The area of the planarizing surface occupied by the voids increases with increasing radial distance to reduce the contact area between the wafer and an abrasive slurry on the surface of the polishing pad towards the perimeter of the pad. Thus, at the periphery of the pad where the linear velocity of the pad is high, the voids reduce the polishing rate of the wafer compared to a planarizing surface without voids.
Although the non-abrasive polishing pad of U.S. Pat. No. 5,020,283 reduces the nonuniformity in polishing rates across a wafer, it may not provide adequate control of the polishing rate to produce a uniformly planar surface on the wafer. The pad of U.S. Pat. No. 5,020,283 seeks to control the polishing rate across the wafer by reducing contact area between the wafer and the slurry at selected areas on the pad. However, the distribution of the slurry between the wafer and the pad may not be uniform under the wafer because the perimeter of the wafer wipes the slurry off the planarizing surface leaving less slurry under the center of the wafer. Thus, even though existing devices control the contact area between the wafer and the pad at selected regions of the pad, they may not effectively control the polishing rate across the face of the wafer.